Field relationships and geological setting

1. Introduction

In recent years there have been great advances in our understanding of the mainly Proterozoic hypabyssal dyke swarms intruded into Precam- brian cratons. Much information has come from studies of mainly Phanerozoic continental flood basalt CFB provinces which have similar chem- istry. Although field evidence of such a relation- ship is often lacking see arguments expressed in Ross, 1983; Tarney, 1992; Cadman et al., 1995, it can sometimes be deduced that flood basalts were fed from the extensive dyke systems which are now exposed in Precambrian cratons e.g. Baragar et al., 1996. Despite very detailed research on both phenom- ena, the degree to which various petrogenetic processes such as fractional crystallisation, crustal contamination and mantle metasomatism control their chemistry is still hotly debated. An obvious difficulty in any petrogenetic analysis of dykes is that the same process may have operated on the magma at different stages in its genesis; for exam- ple, crystal fractionation within a basaltic magma may take place both prior to dyke injection e.g. within a magma chamber and subsequently within the dykes themselves, the relative influence of fractionation within each environment may be very difficult to ascertain. Hence although many studies of intradyke petrogenetic processes have been undertaken e.g. Gibb, 1968; Komar, 1972, 1976; Ross, 1983, 1986; Platten and Watterson, 1987; Blichert-Toft et al., 1992; Ernst and Bell, 1992 the degree to which basalt petrogenesis may be controlled by hypabyssal processes within mafic dykes is still uncertain. Study of the Kangaˆmiut dyke swarm offers an excellent opportunity to help resolve some of these questions. Earlier workers have noted that unlike the vast majority of continental mafic swarms, the dykes were injected into an overall contractional environment e.g. Escher et al., 1976 and throughout much of their extent horn- blende is the dominant primary ferromagnesian mineral Korstga˚rd, 1979; Bridgwater et al., 1995. However, the major element chemistry of the swarm suggests a normal tholeiitic Fe-enrich- ment trend Escher et al., 1975; Bridgwater et al., 1995. As hornblende is associated with calc – al- kaline fractionation, it would appear that the petrogenetic processes governing the chemistry of the dykes may be unrelated to the crystallisation processes within the dykes themselves. The field setting and unusual petrographical characteristics of the Kangaˆmiut dyke swarm also require that models developed to explain the petrogenesis of other dyke swarms are applied in order to test their validity. In this paper we seek to undertake comprehensive major and trace element modelling of the chemistry of the Kangaˆmiut dykes with a view to understanding the processes governing their formation.

2. Field relationships and geological setting

The Kangaˆmiut dyke swarm was emplaced into the high-grade gneisses of the Archaean craton of SW Greenland Fig. 1, occurring in an area spanning 200 km south of and 100 km to the north of the Nagssugtoqidian orogenic boundary Bridgwater et al., 1995. Although originally re- garded as an ensialic orogen, many more recent studies of this orogeny suggest that it took the form of a continent – continent collision between 2.1 and 1.7 Ga Kalsbeek et al., 1987; Marker et al., 1995; Kalsbeek and Nutman, 1996a,b; Kriegs- man et al., 1996. Contacts between the dykes and country rocks are usually sharp with little evi- dence of crustal remelting or absorption at the margins of the dykes. The host rocks for the dyke swarm are mainly tonalitic and granodioritic gneisses. Additionally, there is a tendency for paragneiss to be associated with zones of high strain see Fig. 1. These paragneisses are gener- ally intermediate to highly siliceous in composi- tion and sulphide-rich. Smaller quantities of marbles are also present. The protoliths of the paragneisses are uncertain, but based on our field observation and compositional character, proba- bly consisted of volcano-sedimentary sequences interbedded with small amounts of limestone. Early descriptions of the Kangaˆmiut dykes were published by Ramberg 1948, Noe Nygaard 1952 and Escher et al. 1975. An extension of the dyke swarm is believed to occur in SE Green- land e.g. Bridgwater et al., 1990. Within SW Greenland, the study of fault movement history shows that the dykes are demonstrably younger than the 2.2 Ga high magnesian and tholeiitic ‘MD’ [‘m6etad6olerite’] dykes which intrude the southern part of the craton Berthelsen and Bridg- water, 1960; Hall and Hughes, 1990. U – Pb zir- con SHRIMP analysis dated two the Kangaˆmiut dykes at 2046 9 8 Ma and 2035 9 5 Ma Kalsbeek and Nutman, 1996a,b; Nutman et al., 1999. Re- cent 40 Ar 39 Ar data confirm this 2040 Ma age Willigers et al., 1999. Structural studies by Escher et al. 1976 showed that dyke orientation veered from NNE – SSW to NE – SW approaching the Nagssugtoqid- ian orogenic boundary Fig. 1, with a second subordinate set of ESE-trending intrusions also being present. Escher et al. 1976 suggested that the two sets were coeval, and that they were intruded along conjugate shear fractures. How- ever, later studies of field and cross-cutting rela- tionships suggest three distinct crosscutting sets, the E – WESE – WNW set is oldest, being crosscut by the NNE – SSW and subsequently the NE – SW Fig. 1. Geological Map of West Greenland. Simplified from Marker et al. 1995. Insets: a Map of Greenland showing location of main map area: black = Nagsugtoquidian belt; b Details of sample localities. dykes Mengel et al., 1996. The NE – SW set constitute the main set of intrusions and include the two dykes dated by Nutman and Kalsbeek 1996 at ca. 2.04 Ga. North of Itilleq Fjord, Kangaˆmiut dykes were emplaced into Archaean E – W trending amphibolite facies zones.

3. Mineralogy and petrology